The present invention relates to transceivers for use in a frequency division, two-frequency multichannel communication system having:
a multichannel transmitter, with transmission channel selection means for selecting a wanted transmitter frequency, and a multi-channel receiver, with receive channel selection means for selecting a wanted channel receiving frequency band.
In such transceivers, it is commonplace for the control system of the transmission channel selection means and that of the receive channel selection means to be ganged together so as to simultaneously select the respective transmitter frequency and receiving frequency band of the same two-frequency channel.
In a single frequency, frequency division, multi-channel cummunication system, a plurality of channels are each allocated a single frequency band, each band being centered on a chosen frequency, the channel center frequencies being spaced apart by a given frequency spacing, for example a frequency spacing of 25 kHz between center frequencies.
In a two-frequency communication system, each channel is alotted two frequency bands the center frequencies of which are relatively widely spaced apart, for example a frequency spacing of 5 mHz. In a two-frequency net system, a central station broadcasts to a number of remote stations each station having a transmitter and a receiver. The remote stations may be mobile or portable transceivers. The transmitter of the central station is set to radiate signals at a particular frequency (F1) and the receivers of the remote stations are all tuned to receive signals at the same frequency (F1), whereas the transmitters of the remote stations are set to radiate signals at a different frequency (F2) while the receiver of the central station is tuned to receive signals at that different frequency (F2). Thus, the central station can broadcast to all the remote stations and any one remote station can transmit a message to a central station.
If the receiver output of the central station is connected to the input of the central transmitter, then any signal radiated by a remote station is received by the central station and re-broadcast to all other remote stations. The central station of such a network is commonly referred to as a talk-through base station.
In a communication network employing a talk-through base station and several remote mobile or portable stations, it is a common practice to employ multi-channel transceivers in the remote stations. In such a system, a particular two-frequency channel for example having center frequencies F1 and F2 is reserved for communication between remote stations via the talk-through base station while the remaining two-frequency channels are available for communication between individual stations either inside or outside the network. The two-frequency channel system permits duplex communication between suitably equipped individual stations although only simplex communication is possible when communication takes place via the talk-through base station.
The power output of a mobile station is usually less than that of a base station and the power output of a portable station is usually less than that of a mobile station. To facilitate communications, it is sometimes desirable to provide a two-way interchannel relay station or repeater located within range of a talk-through base station via which a remote mobile or portable station with insufficient range to call the base station direct, is still able to communicate with the base station and hence with other remote stations of the network. Communication takes place between the relay station and the base station on a first two-frequency channel having, for example, the two center-frequencies F1 and F2 while communication between the relay station and the mobile or portable station takes place on a second two-frequency channel having, for example, the two center frequencies F3 and F4.
It will be appreciated that a relay station set to retransmit a signal having the same carrier frequency as that of the received signal could result in severe distortion of the repeated signal. Accordingly, an interchannel relay station is desirable.
Known interchannel relay stations are constituted by two transmitter/receiver combinations in which the receiver and transmitter of each combination is in a "back to back" relationship i.e. the output of the receiver of each combination being connected to the input of the transmitter of the same combination. For instance, assuming a first two-frequency channel having frequencies F1 and F2, at which frequencies a talk-through base station respectively transmits and receives and assuming a second two-frequency channel having frequencies F3 and F4, the receiver of one transmitter/receiver combination of the relay station would be tuned to receive the band center on frequency F1 and the transmitter of the same combination set to radiate at frequency F4 while the receiver of the other transmitter/receiver combination of the relay station is tuned to receive the band center on frequency F3 with the transmitter of its combination set to radiate at frequency F2. In this way, a remote station of insufficient range to communicate with the base station, but within range of the relay station is able to transmit on the frequency F3 and receive on the frequency F4. A message transmitted at the frequency F3 by the out-of-range remote station is received by the relay station and retransmitted by the relay station at the frequency F2. The message retransmitted by the relay station on the carrier frequency F2 is received by the base station and retransmitted by the base station at the carrier frequency F1 so as to be received by all other remote stations of the network. A replay transmitted from the remote station on a carrier frequency F2 is received by the base station and retransmitted on carrier frequency F1 which is received by the repeater, the repeater retransmitting the replay to the out-of-range remote station on the carrier frequency F4.
It is also known to provide an interchannel two-way relay station for repeating between a two-frequency channel and a single frequency channel. For example, with a two-frequency net system comprising a talk-through base station operating on a first channel having frequencies F1 and F2 and several remote stations, the system is sometimes extended so as to include, via an interchannel relay station, one or more stations, for instance portable transceivers, equipped to transmit and also receive on the single frequency F3 of a single-frequency second channel. In this case also, the interchannel relay station is constituted by two transmitter, receiver combination, the respective transmitters and receivers of which are in back-to-back relationship. The receiver of one combination is turned to the band centered on the frequency F1 and the transmitter of that combination is set to transmit at a frequency F3 whereas the receiver of the other combination is tuned to the band centered on a frequency F.sub.3 whereas the transmitter of that combination is set to transmit at the frequency F2.
The desirability of modifying a conventional multi-channel mobile transceiver so as to be able to function as a two-way interchannel relay station when necessary is acknowledged by persons skilled in the art. However, such modification hitherto has required provision of an additional transmitter and an additional receiver together with other additional equipment such as diplexer units and aerial systems to achieve a satisfactory result.